In a frequency domain in the range of 10 MHz to 1 GHz, ferrite, carbon, or the like is mainly used as a dielectric loss material or a conductive loss material for electric wave absorbers. In a frequency domain of 1 GHz or higher, a conductive metal plate, a metal net, a metal fiber or the like is used. These materials are usually combined with a plastic, a rubber, or the like and then used as an electric wave absorber in the form of a sheet.
Recently, in particular, a thin electric wave absorber used for the GHz band has been desired, and various novel materials have been actively developed. Examples thereof include a material produced by dispersing carbon fiber in a calcium silicate molded article (Patent Document 1); a material produced by mixing a powder of magnetoplumbite-type hexagonal ferrite with a holding material composed of, for example, a rubber, a resin, or an inorganic material such as calcium silicate (Patent Document 2); a material produced by dispersing a soft magnetic powder composed of an Fe-based alloy containing 5 to 35 weight percent of Cr in a rubber or a resin (Patent Document 3); a material produced by mixing and dispersing a soft magnetic flake powder composed of a stainless steel SUS 430 with a synthetic resin (Patent Document 4); and a material including an inorganic fiber, a resin binder, and a fiber or a powder having conductivity or magnetism and having a porosity in the range of 35% to 89% (Patent Document 5).
An example of an electric wave absorber including a general building material is an inner wall material for absorbing electromagnetic waves in a band in the range of 70 MHz to 3 GHz, the inner wall material containing gypsum, asbestos cement, or calcium silicate as a main material and a carbon powder, a ferrite powder, a metal powder, a metal compound powder, or a mixture thereof, which is an electromagnetic wave loss material (Patent Document 6).
Examples of known woody electric wave absorbers include an absorber produced by joining a small pieces of electromagnetic wave shielding material with a woody material using an adhesive (Patent Document 7) and an absorber produced by mixing a carbon powder or a carbon fiber with wood chips (Patent Documents 8, 9, and 10). The present inventor has developed a magnetic woody material, which is a novel building material, having functions such as magnetic absorbability and electric wave shielding (Patent Document 11 and Non-Patent Documents 1 to 3).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-283971    Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-354972    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-200990    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-274587    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-60381    Patent Document 6: Japanese Unexamined Patent Application Publication No. 6-209180    Patent Document 7: Japanese Unexamined Patent Application Publication No. 61-269399    Patent Document 8: Japanese Unexamined Patent Application Publication No. 1-191500    Patent Document 9: Japanese Examined Patent Application Publication No. 6-82943    Patent Document 10: Japanese Examined Patent Application Publication No. 6-85472    Patent Document 11: Japanese Unexamined Patent Application Publication No. 2001-118711    Non-Patent Document 1: Oka, Jisei mokuzai no kiso tokusei (Fundamental characteristics of magnetic woody materials),    Nihon Oyo Jiki Gakkaishi (Journal of Magnetic Society of Japan), Vol. 23, No. 3, pp. 757-762 (1999)    Non-Patent Document 2: Journal of Applied Physics, Vol. 91, No. 10, Parts 2 and 3, 15 May, pp. 7008-7010 (2002)    Non-Patent Document 3: New Scientist, 29, June, p. 20 (2002)